Bond-testing (BT) equipment, such as the ONDT Bondmaster1000e+ using pitch-catch probes, offer a method for one sided non-destructive inspection of honeycomb sandwich composite materials. The pitch-catch (P-C) probe generates acoustic waves in the part using a first tip pressed in contact with the surface, and reads vibrations using a second tip simultaneously in contact with the surface. The probe can then detect defects through changes in vibration amplitudes and/or changes in propagation delays (phase) from generating tip to receiving tip.
The detection capability of a given pitch-catch (P-C) bond-testing (BT) non-destructive instrument is directly related to the nature and size of a given defect and the frequency used to detect the defect. Composite honeycomb sandwich structures can present various types of defects ranging from skin to core disbond or crushed core (disbond) and skin to skin disbond (delamination). A typical repair artifact referred to as potting can also be detected with the P-C technique. The relation between the detection capability and the relation between the defect size and test frequency is notable for small defects which are detected at a limited range of resonant frequencies. Large defects can typically be detected over a larger range of test frequencies.
Disbond type indications are typically detected by an increase in signal amplitude for a given frequency. Delamination and potting type indications are quite different in that the amplitude for such indications remains relatively stable; however, the phase of the return signal varies due to the effect on the acoustic wave speed caused by these indications. Delamination type defects lead to a decreased wave speed whereas potting type defects lead to an increased wave speed.
As the optimal test frequency depends on the test part structure (i.e. number of plies, ply type, core thickness, core type) and on the defect type, shape and size, selection of a single frequency for inspection can be troublesome.
Yet another drawback of the background art is its inability to distinguish between various defect sizes. In fact, there are some test situations when a smaller defect can generate a stronger signal than a bigger defect at a given frequency.
To ensure a good possibility of detection (POD) for a range of defect sizes including small defects during a BT inspection, several frequencies must be used. These test frequencies should fully cover the frequency range over which real defects are likely to occur in a given sample.
To provide inspection data for multiple inspection frequencies, most background art units offer a “sweep” inspection mode which sequentially alternates through a band of excitation frequencies. However, data acquisition goes through a single set of test parameters (gain, phase angle, etc) to be displayed in a unique impedance plane. This method has important limitations resulting from the non-linear probe response over its operating frequency range. As presented in FIG. 3, the probe response over a good portion of a test sample varies over the frequency range. There is therefore an inherent disadvantage to using the same inspection settings for all test frequencies and displaying the results from all test frequencies in the same impedance plane. Detection of phase shift using the “sweep” mode is also nearly impossible.
Another disadvantage with the background art is that few systems offer a means for representing BT data in a C-scan. Most currently available systems provide manual inspections but cannot provide a two-dimensional mapping of the test sample. Although representing impedance plane data into a readily interpretable C-scan image is provided by the background art for a limited range of frequencies, means for combining data from a relatively large range of multiple frequency C-scans into a combined C-scan without negatively affecting POD and signal-to-noise ratio (SNR) is not known to be an available solution.
Considering the aforementioned drawbacks and disadvantages, there is a great need for a multi-frequency bond testing instrument with a data representation and test method having the ability to meet the following objectives: a) represent the BT amplitude at multiple test frequencies on a single C-scan image; b) compensate the non-linear behavior of the BT probe over the frequency range; c) differentiate defects of various sizes, improve or at least maintain the signal-to-noise ratio observed when scanning at specific frequencies; and d) detect delamination type defects and differentiate similar signals originating from potting.